Gammaroid Aetosaurs of Lake Baikal

Containing 20% of the world’s freshwater, reaching a depth of over 1600 meters, and having endured for 28 million years, Lake Baikal is the deepest, most voluminous, and most ancient of all freshwater lakes (Mats, 1992; Logatchev, 1993). Situated north of Russia’s Mongolian border, the “Blue Eye of Siberia” is not wanting for motes; Baikal’s species assemblages include reefs of green, branching sponges, a landlocked seal (the nerpa, Pusa sibirica), and spectacular adaptive radiations of cottoid sculpins and gammaroid amphipods. I could go on (and, in later posts, will), but let it suffice to say that the bed and waters of this lake are surreal beyond imagination.

These last, the gammaroids of Baikal, represent a burst of diversification without peer amongst contemporary freshwater invertebrates (Brooks, 1950; Kozhov, 1963). Of some 800 gammaroid species known worldwide [EDIT: see comments for discussion], over one third are endemic to this lake (Kamaltynov, 2002; Vaïnölä et al., 2005). While gammaroids elsewhere tend to be morphologically conservative and rather nondescript (if you’ve seen one scud, you’ve seen ’em all), the Baikalian taxa are highly diverse in body form and ecological niche, ranging from “dwarfed herbivorous Micruropus to giant deepwater Abyssogammarus and Garjajewia” and “from the parasitic genera Pachyschesis and Spinacanthus to the pelagic Macrohectopus” (Sherbakov et al., 1998).

Some, in fact, bear spiny protrusions of varying form and position (termed “body teeth”), which presumably serve a defensive function. The Baikal forms are close to unique in having these armaments (Barnard and Barnard, 1983) — as far as gammaroids go. Aetosaurs are a different matter.

Darren Naish, at Tetrapod Zoology, presents a far better overview of these “omnivorous armoured crurotarsans of the Late Triassic” than I could hope to give — well worth a read even if you’re not a vertebrate person (i.e., a “spineless scalawag”). Despite being 6 m long instead of 6 cm, having osteoderms in place of chitinous plates, and — let’s face it — being a Texan archosaur instead of a lacustrine crustacean from Siberia, Desmatosuchus haploceros bears a striking resemblance to the Acanthogammarus shown below.

I’ll concede that dorsal protrustions are more pronounced in our amphipod than in our aetosaur, but the shapes and relative lengths of the curved lateral spines are remarkably similar in both.

Acanthogammarus, Sherbakov et al. explain, are benthic, mostly carnivorous (though not above scavenging and detritivory), and represented in both shallow (<200 m) and deep water by large (20-50 mm) and giant (>50 mm) species. A. victorii is strikingly colored (those of you familiar with the BBC’s “Planet Earth: Freshwater” may recall the orange fellows beneath the ice) and I suspect this to be the true identity of the aetosaur-impersonators above.

[All photos of amphipods taken from this page; first aetosaur photo from here; second from here.]

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This is awesome. I never knew that Lake Baikal was host to such a diversity of Gammarids.

I have a question. From other sources I’ve read that the number of described Gammarid species is closer to 5700, rather than 250. Which level of classification of Gammarids are you talking about, the suborder Gammaridea, or a particular family of that suborder?

Thanks for pointing out the inconsistency — I’ve corrected the text and the title. As it turns out, the gammar[-ideans, -oids, everything but plain old -ids]s of Lake Baikal are even more diverse than I’d thought.

As of 2005 (Vaïnölä et al.), Amphipoda was represented in freshwater by 1,870 species or subspecies (and by 9,100 in all habitats). Of these continental forms, the Gammaroids (Suborder Gammaridea, Superfamily Gammaroidea) accounted for 800 species. The family Gammaridae currently has only 304 species; it was split extensively … to say the least.

Lake Baikal is now thought to boast 363 endemic species and subspecies of amphipods, with 12 more in the downstream watershed — all in all, 72 genera and 7 endemic families (Kamaltynov, 2002).

Vaïnölä provides this breakdown of Baikal’s endemic Gammaroid families (number of species in parentheses): Acanthogammaridae (159), Baikalogammaridae (1), Eulimnogammaridae (122), Macrohectopidae (1), Micruropodidae (55), Pachyschesidae (16), Pallaseidae (22). That seems to be one more than the 363 specified by Kamaltynov, so I guess one count or the other is in error.